Cellulose is a natural polymer having the widest distribution and the largest reserves in nature, and is an inexhaustible renewable source. Compared with synthetic polymers, cellulose has advantages such as complete biodegradability, non-toxicity, non-pollution, good biocompatibility, etc. With the daily exhaustion of oil, coal and other fossil resources, the research and development on cellulose-based materials are of great importance for the promotion of sustainable human development.
Regenerated cellulose film (also known as cellophane, cellulose film) prepared from cellulose as raw material is an important class of film material. The film has the features such as being non-toxic, transparent, antistatic, degradable, has good barrier and high temperature resistance, and has been widely used in the packaging of products such as food, pharmaceuticals, cosmetics, high-end clothing, precision instruments, etc. Because natural cellulose has high crystallinity and a number of inter-molecular and intra-molecular hydrogen bonds, it cannot be molten and is hard to be dissolved, resulting in poor processing performance. In industrial scale, the existing production processes of regenerated cellulose films mainly use viscose technology. After derivatization of natural cellulose, a cellulose solution is prepared, and then cellulose is regenerated. In the production process, not only the problems of high pollution and high energy consumption occur, but also cellulose degradation is severe during production, therefore it is difficult to improve the product performance.
Biaxial stretched technology has been widely used to improve physical and mechanical properties of polymer films. After nearly 30 or 40 years of rapid growth, it has formed a high production capacity, and products include a variety of synthetic polymers, for example, polypropylene, polyester, polystyrene, polyvinyl chloride, polyamide. Compared with non-stretched films, biaxially stretched polymer films have significantly improved mechanical strength, improved transparency and surface gloss, uniform thickness and small thickness deviation; improved barrier properties. However, the biaxial stretched technology requires polymer raw material with certain specific properties, for example, polymers should have high strength (see Development of BOPP Specific Material, Yisen Li, China Synthetic Resins Plastics, 1991, 8 (4): 25)
In the production of regenerated cellulose films by viscose technology, films can achieve high strength in longitudinal direction by adjusting the speed ratios among different rolls. In contrast, the cellulose films have a poor strength in transverse direction, resulting severe shrinkage. Because the molecular weight degradation of cellulose in the derivatization process is severe, the transverse tensile strength of the cellulose films produced by viscose technology cannot be improved by stretching transversely.
In a solvent system for non-derivatized cellulose, such as N-methylmorpholine-N-oxide (NMMO), the molecular weight degradation of cellulose is reduced during dissolving and regenerating. The obtained films can be stretched in a transverse direction. Therefore, it is possible to prepare biaxially stretched cellulose films.
PCT Application WO97/24215 provides a method for manufacturing a biaxially stretched regenerated cellulose film from a solution of cellulose in NMMO. By the method, the cellulose solution is extruded and adheres to an elastomeric film. Before the NMMO solvent is removed, the elastomeric film is stretched in order to biaxially stretch the solution of cellulose. However, because of the low strength of the solution of cellulose, the liquid film could easily crack during stretching, causing instability in the process. In addition, the stretch-oriented molecular chains in the solution could easily restore, and the shaped regenerated cellulose film may further shrink in the washing and drying processes, which could influences the improvement of the performance of the cellulose film. In PCT Application WO98/49224, a solution of cellulose in NMMO is extruded from an extrusion die into a coagulation bath. After washing out the NMMO solvent, the film is stretched transversely, and finally dried to be shaped; or after wetting the dried film, the film is stretched transversely. However, the transversely stretched film during drying can still have a certain degree of shrinkage, resulting in decreasing the size and the tensile strength of the film in the transverse direction. Therefore, based on the PCT application WO 98/49224, in PCT application WO02/100925, transverse stretching is performed in two steps when a cellulose film is formed after passing through a coagulation bath and NMMO is washed out, wherein in the first step the film is excessively stretched, and in the second step the stretched film is loosen to the desired stretching degree. In PCT Application WO02/100926, the transverse shrinkage of film is reduced by continually maintaining a certain tension on a transversely stretched film in the transverse direction. Although the obtained stretched film maintains a high dimensional stability when exposed to water or alkali solutions, the tensile strength of the stretched film in the transverse direction does not significantly increase and is less than 60% of the longitudinal strength, resulting in unbalanced performance in the transverse and longitudinal directions.
Recently, ionic liquids as a new class of green solvents for natural cellulose have attracted much attention, due to their strong dissolving ability, nonvolatile nature, good chemical and thermal stability, and easy recycling. There are patents about ionic liquids used to dissolve cellulose (see Chinese invention patents: ZL 02147004.9, ZL 02155945.7, ZL 02823875.3, ZL 200680012598.X, CN 200710085298.0). A patent also reports ionic liquids as a solvent for the preparation of regenerated cellulose film (see ZL 200410101800.9): a shaped solution of cellulose in ionic liquids enters into a coagulating bath to coagulate and regenerate. It is further washed and dried to give a cellulose film.
Currently, there has been no report about the preparation of biaxially stretched films by stretching transversely in the washing process after cellulose coagulates into a gel film. Our research has found that a solution of cellulose in ionic liquids after coagulation forms a cellulose gel film with excellent gel strength. Meanwhile, because a cellulose gel film containing an appropriate amount of ionic liquids has better deformability than that without the cellulose solvent which is completely removed, the former is more suitable for the biaxial stretching process; and the transversely stretched gel film could has increased width and decreased thickness, which are advantageous for washing the residual ionic liquid solvent in the film out and removing water from the film when drying. In addition, there has been either no report about the preparation of biaxially stretched cellulose functional films.